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TG Worms Duel With Penises
By Science News
Contributed by Ginamarie
February 14, 1998
Talk about a battle of the sexes. Researchers have found
hermaphroditic
flatworms that rear up, expose their stubby penises, and literally
duel.
Hermaphrodite flatworms mate in pairs, dueling to stab a penis
anywhere on a
partner's body while avoiding being stabbed.
In bouts that can last 20 minutes to an hour, marine worms of the
suborder Cotylea feint
and writhe for position. Each attempts to stab its penis into an
exposed area of its sex
partner's body while avoiding getting jabbed itself.
A worm that scores a hit injects sperm into whatever region of
flesh it penetrates, report
Nicolaas K. Michiels of the Max Planck Institute for Behavioral
Physiology in Starnberg,
Germany, and Leslie J. Newman of the Smithsonian Institution in
Washington, D.C.
After the strike, sperm stream through the partner's body tissue,
creating pale streaks like
lightning jags on their way to fertilizing eggs in the ovaries.
Researchers knew that these flatworms inject sperm into random
areas of
flesh, but "penis fencing is new," Newman says. No one had
described the
dueling until she and Michiels spent some 20 hours continuously
watching
pairs of captured worms mating in old ice cream containers. The
details for
one moderately aggressive species appear in the Feb. 12 Nature.
"It's better to stab than to be stabbed," says Newman. The stabber
fathers
offspring without the energy drain of healing torn flesh or
producing eggs.
"The interesting thing," says Michiels, "is that hermaphroditic
partners run
into conflicts because they usually have identical but incompatible
interests." People may not realize how simple many human sexual
conflicts
are in the grand scheme of nature. "Once a male and a female decide to
mate, there is no discussion about who will give and who will
receive
sperm," Michiels says.
The dueling worms illustrate one extreme of hermaphroditic
difficulties -- both partners vying for the male role -- but other
flatworms have
the opposite problem. "Individuals have to 'beg' to receive sperm,"
Michiels
says. In these species, hermaphrodites project what looks like a
penis but
reaches out to a partner's male organs and withdraws sperm.
Hermaphrodites, which share the same sexual interests and strategy, may
be more likely to evolve physically damaging sex, Michiels
speculates. For
the marine flatworms, particularly aggressive duelers may produce
more
offspring than so-so stabbers. "It results in a kind of
escalation," he says.
In two-sex species, the females have a strong interest in not
getting their
flesh ripped to pieces during mating and so may avoid injurious
males.
Natural selection will favor males that are "well-behaved,"
Michiels says.
Jabbing sperm directly into flesh may have developed to circumvent
female
devices to control fertilization or wrest some unintended benefit
from it,
Michiels speculates. A number of species with set routes for sperm
have
special female adaptations so that "most of the sperm go straight
into the
gut."
Michael Siva-Jothy of the University of Sheffield in the United
Kingdom says
that among species studied so far, "I think the dueling and the
overtness of
the dueling are quite unusual." He adds that he would not be
surprised if the
worm report inspired closer observations of other hermaphrodites,
whose
romances may turn out to be just as weird.
Another specialist in mating conflicts, William G. Eberhard of the
University
of Costa Rica in San Jose and the Smithsonian Tropical Research
Institute
in Panama City, Panama, urges researchers to follow the sperm. He'd like
to know whether the stabbed worm digests or otherwise manipulates
the
sperm it receives. He warns against a common bias: "Females are
generally taken as relatively passive."
Eberhard also points out the difficulty of untangling the interests of
the
sexual combatants. Is a dodging partner just filtering out lousy
duelers?
Then, he says, the traditional battle of the sexes becomes
"selective
surrender."
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